High frequency electronic oscillators, broadly in the range of 10 to 100 MHz, are used for a variety of purposes ranging from radio transmission and reception to heating of materials. A particular application is for the formation of a plasma discharge with an apparatus known as an induction coupled plasma (ICP) generator. Such plasma systems typically are used for spectroscopy, treatment of fine powders, melting of materials, chemical reactions and the like. These applications derive from the high temperatures inherently associated with a plasma, which are high enough to effect electron excitation and ionization of the plasma gas and injected materials. Plasma generation also is utilized to produce ions utilized in instruments for mass spectroscopy and, at low pressure, for vapor deposition processes. Other applications are associated with induction heating of materials, such as graphite of an induction furnace, melting of metals, and the like.
Oscillators must be capable of generating sufficient power for the application, i.e. approximately one kilowatt for ICP. Generally this has been accomplished with circuits incorporating vacuum tubes. Typical tube oscillators in current use with ICP are shown in U.S. Pat. Nos. 4,629,940 and 4,818,916, and copending patent application Ser. No. 08/079,963 filed Jun. 18, 1993 now abandoned and its counterpart European patent application publication No. 0 568 920 A 1, all commonly owned by the present assignee. Other conventional tube oscillators, some of which may not necessarily be suitable for induction heating, are taught in standard texts on the subject. Tube oscillators in common use for ICP tend to be cumbersome, sensitive to tuning problems and limited in efficiency of power transfer (up to about 50%). Tuning problems are associated with requirements for alignment of impedance matching networks, and with the coupling with the ICP and the variable transfer of power thereto. Tubes tend to have changing characteristics with time. Tube circuits also require periodic replacement of tubes, due to finite lifetime of the thermionic emission of the filament electron source.
Oscillators based on transistors have been known for applications such as radio, audio and the like but, to the knowledge of the present inventors, the configurations described herein have not been used for plasma generation. Examples of transistor oscillators are taught in a publication "High Frequency Circuit Design" by J. K. Hardy, 246-252(Prentice-Hall 1979). These generally deliver low power and use parallel oscillator circuits.